Photomechanical color reproduction



Patented July 16, I935 UNITED STATES PATENT OFFICE PHOTOMECHANICAL COLORREPRODUCTION No Drawing. Application September 9, 1933,

Serial No. 688,828 g Claims.

This invention relates to photographic reproduction in color and moreparticularly to improvements in color reproduction by a photomechanicalprocess whereby more accurate results can be obtained than is possiblewith'the processes at present known.

In photomechanical process work the sketch to be reproduced isphotographed through appropriate filters upon a series of sensitiveplates to provide a like number of color separation images from whichthe printing plates are prepared in any Well known manner.

It is well known that the visual or subjective brightness of standardpalette colors differs substantially from their actinic or photographicbrightness and that the color separation images obtained from a sketchmade with these colors will not produce in the final print correct colorrendition even with the best contemporary printing inks. In practice,this deficiency is generally combatted by retouching the photographs andworking the plates, which practice requires highly skilled artisans andis both costly and time consuming.

It has been proposed to modify the colors of the original sketch byapplying dyes on the surface or on a superimposed film, so as to falsifytheir visual values and/or reflecting powers in such a manner as tocompensate for the deficiencies in the process as it has been realizedin practice, which are due to the limitations inherent in the 'manner ofprinting, and in the available pigments that must be used for making theoriginal and for compounding the printing inks. However this is merely amethod of interposing hand retouching at a different point in thereproduction process.

Other related methods place a great burden upon the artist and thepurchaser by distorting the visual appearance of the sketch so that hemust think in terms of one color value and paint in a different colorvalue. As a consequence of my invention, the original sketch iscorrected without handicapping the artist or appreciably distorting thevisual appearance of the sketch due to the fact that the correctingprocedure .of my process is mainly technical rather than artistic.

It is an object of my invention to provide special coloring materialsfor creating an original to be reproduced by a photomechanical process.

Another object of my invention is to provide a method whereby colorseparation images may be made from originals made with the specialcoloring materials of my invention.

Other objects and advantages of my invention will appear from thefollowing description and its novel features are pointed out in theappended claims.

The special coloring materials of my invention 5 are obtained byincorporating in certain of them luminous or fluorescent material inratios such that suflicient actinic light will be available to exposethe photographic material the desired amount without over-exposing theother parts of the original. The fluorescent material may beincorporated in any suitable Way as by dissolving it in any solventwhich is miscible with the vehicle of the paint and then mixing it withthe various colored paints by rapid stirring, or the fluorescentcompound may be mixed dry with a given pigment and the mixture thenground with the vehicle, or the pigment particles may be coated with thefluorescent compound, as from a solution in a volatile solvent, or byprecipitation. After removal of the solvent, there remains a dry,fluorescent, pigment powder, that can be used in paints, pastels,crayons, carbon tissues, etc. Any desired fluorescent material may beused, namely, florescein, B-naphtholdisulfonic acid, sodium salicylate,luminous alkaline-earth sulfides, anthracene, and I have found diethyldihydrocollidine-dicarboxylate especially satisfactory for this purpose.The quantity of fluorescent material which should be used depends, amongother things, upon the light source, the panchromatic emulsion, thetechnique and the coloring materials used. In general the quantity willbe determined by the additional luminosity needed to effect colorcorrection as above described and ordinarily the red coloring materialswill require a smaller quantity than the blue and the yellow willprobably require none at all. Again, some pigments have more or lessfluorescence of their own. This must be taken into account by addingenough of the fluorescent compound to supplement the naturalfluorescence, and raise it to the required level of intensity.

For three-color photographic prints, such as are made by modificationsof the carbon and imbibition processes, three pigments or dyes are usedof the following hues: yellow, bluish-red, and greenish-blue. As givinga more extended explanation of my process, however, it is moreenlightening to consider the problem of painting a commercial sketch orpicture in full color. For multi-color painting, a set, or palette, ofpigments must be provided that will satisfactorily cover the colorscale. As an example of a typical minimum palette for such purposes Imight list ultramarine blue (colloidal sulfide), chromium oxide green(chromium oxide), hansa yellow (Schultz and Lehmann No. 84), cadmiumorange (cadmium sulfide), permanent geranium lake (General DyestuifsCorp), carbon black (carbon), and titanium oxide white (titanium oxide).

In the reproduction of ultramarine blue by the contemporary 4-colorprocess, a normal color separation gives one practically as much red asblue, and a considerable amount of yellow and black, so that the actualratios of printing inks printing together to make ultramarine blue wouldbe of the following order: blue 90, red 80, black 50, yellow 15. Ascraftsmen know, this gives a purplish-black. The current practice is toadjust the various densities by laborious and diificult hand retouchingin one form or another until the ratios are roughly, blue 90-100, red15-20, black 0-5, and yellow 0. Ultramarine blue has no naturalfluorescence, but by compounding fluorescent material with it, it ispossible to make supplementary exposures to the fluorescent light andbuild up the densities of all parts containing that pigment until therequired density is reached on the separation negatives. If the exposurewere exactly made, and if no improvements were required over theoriginal, all need for retouching would be removed, but, in any case,the major part is easily eliminated.

The chromium oxide green would be rendered in normal 4-color negativesas roughly, blue'50, red 30, black 40, yellow 40, which with printinginks would give a dark gray with a mere suggestion of blue-green. It isnecessary to reduce the ratios to 0-5 in the case of the red, and 5-8 inthe case of the black, which can be done by supplementary exposures ifthe green is also made fluorescent.

It is convenient to make the brightnesses, or intensities, of thefluorescences in the blue and green proportionate, so that in making thered printing negative, which requires the most correction, the plate canbe exposed to the full fluorescence without having to use a filter tomodify either the blue or the green.

The permanent geranium taken in this example has a natural redfluorescence. This is augmented so as to maintain the fluorescence ofpurples made by mixing the 'red with the blue. Permanent geranium is abluish-red.

fIn the limited palette given in the present example there are no othercolors'requiring correction. It will be seen that high fluorescence isonly required in those pigments requiring process blue ink in theirreproduction, especially if the separate-exposure method, describedbelow as Procedure A," is followed.

A sketch made with coloring materials containing a fluorescent compoundwould not appear distorted in ordinary illumination. Such sketchesshould have some identifying mark or label, but even if not marked theyneed only be viewed in ultra-violet radiation, when the fluorescence, ifpresent, will be plainly visible. It might be well to note that thesketch should be painted on a support having little or no fluorescencein ultraviolet light. On the copyboard with each original it will behelpful for the operator to have register marks, a scale of grays, and afluorescent guide card carrying patches of blue-violet, green, and red,which may be used as described later.

For the first exposures, white flame arcs or any other source of whitelight may be used as at present. For the ultra-violet exposures, mercuryvapor tubes made of ultra-violet transmitting,

white-light-absorbing glass may be mounted at either side and/ or aboveand below the copyboard.

The white light exposures which will be referred to as color separationexposures will be made through the proper filters and should besufficient to record the white of the gray scale as full density. Aftereach one, and without disturbing the plate, the white light source isextinguished and the ultra-violet is turned on. The camera, ultra-violetlamps and copyboard should be suitably enclosed to exclude extraneouslight while the fluorescence exposures are made. These will be referredto as the supplementary," or booster, exposures. The following is aspecific example of how a supplementary exposure may be made. Theultra-violet source is provided with filtering means to absorb thevisible wave lengths between 400 m and 700 m and transmit wave lengthsbetween 350 m and 400 m The original is then reflecting theseultra-violet rays, which are invisible, plus the fluorescence theyexcite, which is visible. In order that the ultra-violet radiation shallnot affect the photographic plate, it is necessary to place a filterover the lens that absorbs between 350 m and 400 my, and transmitsbetween 400 m and 700 m l such as the Corning glass filter known asNoviol-O. A plate exposed in the camera now, through the Noviol filter,will record the fluorescent regions, but will see all the other parts,including whites, yellows, etc., as black. Thus it becomes possible tobuild up density in the negative in certain colors without affecting theremaining colors, or

the grays or whites.

The first exposure is a perfectly normal color separation exposure, andif the negative were developed at that point it would be a normalnegative, with a density in the whites of say 1.4 to 1.5 aftersubtracting fog.

To make the yellow printing negative, a normal first exposure is giventhrough a blue-violet filter. The supplementary exposure is given to thefluorescent image through the Noviol filter, plus a blue-violet filter.This removes excess yellow from the colors containing blue-violetwithout appreciably decreasing the amount of yellow in the greens. Thisis possible because when the fluorescent compound is combined with theblueviolet (ultra-marine) and green paints, the resulting fluorescencetends to assume the hue of the pigments, and by using a blue-violetfilter, the fluorescence from the ultramarine is transmitted, but thatfrom the green is absorbed.

To make a corrected black printing negative it is necessary that all(subjectively) saturated colors should photograph as white. This hasalways seemed a photographic impossibility, but

it has now been accomplished. A normal first.

Procedure B In Procedure A, separate exposures are given to copyilluminated separately by lamps of different characteristics. InProcedure B, simultaneous exposures are given to the copy illumihatedsimultaneously by lamps of different characteristics. A white lightexposure through a filter may take place simultaneously with an exposureto ultra-violet excited fluorescent light through the same filter.

As an example of simultaneous color-separation and color-correction byProcedure B, the copyboard is irradiated with filtered ultra-violet frommercury vapor lamps and at the same time illuminated with white light ofsuch intensity that in the time required to make an exposure to thenormal light the excited fluorescence will build up the density requiredicr color correction. The exposure for a corrected yellow negative wouldbe made under the above conditions through a filter transmitting in theblue-violet only.

Procedure C In this procedure the copy is illuminated by radiation froma homogeneous or mixed source with spectral characteristics differingfrom those of the correcting or supplementary luminescence of thecolors, and by means of filters placed between the original and theplate, say in front of the lens, an effective fraction of theluminescence is allowed to pass, while the larger part of the excitingradiation reflected from, or transmitted by, the original, is absorbed,a residual fraction of the latter being used to make the simultaneousexposure to the reflection values of the original. Thus, with a sketchpainted with black, white, and fluorescent red, yellow, green, andblue-violet, pigments, illuminated with unfiltered mercury vapor light,a corrected black-printing negative is made in a single exposure with afilter transmitting red and blue-green, but absorbing the strong mercurylines in the 405 and 435 m regions and those at about 546 m and 578 mEnough of the blue and green fluorescence will come through the gap inthe blue-green to make an effective exposure, and the fluorescence ofthe red and yellow will be visible through the red transmission band ofthe filter. The density of the filter is low enough that thewhite-to-black values of the sketch are recorded on the negative at thesame time. Procedure C requires the correcting luminescence of thepigments to be diiferently distributed as compared with pigmentsintended for Procedures A and B.

By the blue portion of the sketch is meant all those parts requiringsome process blue ink to match the hue with printing inks.

In the booster exposure on the green filter negative, enoughfluorescence exposure with a Noviol or similar filter should be given tomake the green patch on the guide card equal in density, the densityproduced by the non-fluorescent white of the surrounding card in thewhite light exposure. In making the blue-violet filter negative, thebooster exposure should bring the density of the blue-violet patch up tothat of the white card. In making the black printing negative, thesupplementary exposure should make the blue-violet, green, and red,patches equal in density to white. In making a purple or violet printingnegative, the booster exposure should make the density of the greenpatch equal that of the white card in the negative.

In making color separation negatives from this sketch, severalalternative techniques or modifications of the process are possible, ofwhich three are here outlined as follows:

Procedure A In making the red printing negative, the sketch isilluminated with carbon are light and exposed on a panchromatic platethrough a green filter for a predetermined period, such as A seconds;the sketch is then illuminated with exciting light such as obtained froma mercury vapor source with an ultraviolet transmitting filter and afurther exposure is made through an ultraviolet absorbing filter, suchas a light yellow filter, for A times B seconds where the Factor B hasbeen predetermined to compensate for the low reflection of commerciallypure blue pigments. It will be appreciated that this compensation iseffected by the increased exposure from the blue portion of the sketchdue to the fluorescence of the material therein when excited by theultraviolet illumination.

Ordinarily no correction is necessary on the blue printing negative andit is made in the usual way without any supplementary exposure to thefluorescent image as excited by ultra violet light.

The necessity of a supplemental exposure as above described is due tocauses suggested above. The chief of these, and onezthat has receivedsome quantitative study in the past, is the fact that the green, theblue-green, the blue, the violet, and the purple, of the best availablecommercial coloring materials, have much lower reflections in thosewavelengths which they reflect most, than the yellow, the orange, andthe red. Since the compounding of printing inks depends upon the knownpigments just as much as the manufacture of artists colors, it is nomore possible to get full saturation throughout the color scale in theone than in the other, unless, or until, there is a distinct advance inthe knowledge of the manufacture of pigments. In this respect it mightbe pointed out that at present a blue of 60 per cent reflection in itswave-length region of maximum reflection would be subjectively acceptedas a color of high purity".

Loss of saturation appears to be inherent in the contemporary processesof printing, themselves. It apparently results from absorption of theink vehicle by the paper, and color changes in some vehicles duringdrying. In the case of half-tone screen printing from relief andplanographic forms, which at the present time embraces the bulk of allcolor printing, the color mixture is partly additive, partlysubtractive, and since the theoretical basis of the process issubtractive color mixture, the extent to which additive color mixtureenters in represents loss of saturation in some mixtures, increase inothers, and hue changes in others.

Another complicating factor is that process inks vary in transparency.Yellows are opaque with high reflection, blacks have low reflection buthigh hiding-power, while blues and reds can be obtained with hightransparency.

In the present state of the art, experience and reference to actual,printed, halftone color scales, are the only guides to the results ofcolor mixture in these processes.

No attempt is here made to give examples of exact exposures and the likefor to do so would make it necessary to specify in detail the propertiesof the coloring materials used in making the sketch, the characteristicsof the photographic emulsion and the spectrophotometric absorptioncurves of the filters used. Such detailed specifications would not aid aworker in this art to practice the invention since each worker wouldhave to dei ermine by experiment the corrections necessary to obtain thebest results with the materials he has available.

An important feature of this process is its flexibility andadaptability. In the event of future improvements in the spectralquality of process inks it will only be necessary to decreaseproportionately the color-correction exposures. Similarly. one is notconfined to any specific inks at any time, but the proper colorcorrecting exposures must be established for the inks used in a givencase.

It is to be understood that phosphorescent material may be used for theluminous substance instead of fluorescent material in which case thesupplementary exposure would be made in the dark and a relative longexposure would be necessary.

While my invention has been described as applied to photomechanicalprocess work it is obvious that it may be used wherever color separationimages are to be made from subjects which are originally prepared forthis purpose, or which are of a nature such that they may easily bealtered as required by my invention. Such alteration may readily takethe form of retouching the subject with a transparent or translucentfluorescent solution.

In the claims, the term color saturation is to be read as meaning thepercent reflection or transmission of the characteristic hue. Thus lowsaturation colors denotes colors having relatively low percents ofreflection or transmission of their characteristic hues.

What I claim as new, and desire to secure by Letters Patent of theUnited States is:-

1. The method of producing color separation images for use in aphotomechanical color printing process, which comprises creating theoriginal sketch or color photograph with coloring materials differing incolor saturation and having incorporated in certain of the colors aquantity of fluorescent material, successively exposing a series ofsensitized layers to light reflected by said original with a suitablefilter interposed and giving a supplemental exposure to certain oi saidsensitized layers while exposing the original to radiation of awave-length which excites the fluorescent material and positioning infront of the sensitized layer a filter which absorbs light of the wavelength of the exciting light.

2. In a photomechanical process the method of making a color separationimage corrected for diflerences in the efliciencies of the severalcolors appearing in the original sketch or color photograph whichcomprises incorporating in the low saturation colors a quantity offluorescent material and, in addition to the ordinary exposure, making asupplementary exposure to light emitted by the fluorescent material whenexcited.

3. The method of producing a corrected color separation negative of anoriginal color sketch which comprises exposing a photographic layer tothe natural reflection of the coloring materials in the original and toseparable radiation of luminescent material associated with certain ofthe coloring materials.

4. The method of making a correctly separated record of the red from amulti-colored original in which the colors requiring process blue ink intheir reproduction contain a quantity of fluorescent material whichcomprises exposing a sensitive layer first to visible light reflected bythe original and then to light emitted by the fluorescent material uponbeing irradiated with exciting radiation.

5. In the art of producing polytone prints, the method of predeterminingthe tones of the printing plates to correspond to the process inks to beused which comprises creating the camera copy therefor with paintscontaining fluorescent material in predetermined amounts which are basedin part upon the reflecting powers of the several paints, makingordinary color-separation exposures from said copy and in additionsupplementing at least one of the color-separation exposures with anexposure through a proper filter to light emitted by the fluorescentmaterial.

ALEXANDER MURRAY.

